Inorganic photoconductive substances such as selenium, zinc oxide, cadmium sulfide, and silicon have hitherto been used extensively in photoreceptors for electrophotography. Although these inorganic substances have many merits, they had various drawbacks. For example, selenium has drawbacks that it necessities difficult production conditions and that selenium is apt to crystallize with heat or mechanical impact. Zinc oxide and cadmium sulfide have problems concerning moisture resistance and mechanical strength and further have a drawback that these substances deteriorate in suitability for charge or exposure by the action of a dye added as a sensitizer, resulting in poor durability. Silicon also necessitates difficult production conditions and further necessitates use of a highly irritant gas, resulting in a high cost. Silicon is sensitive to moisture and, hence, care should be taken in handling. In addition, selenium and cadmium sulfide have a problem concerning toxicity.
Organic photoreceptors which employ various organic compounds and in which those drawbacks of inorganic photoreceptors have been mitigated are in extensive use. The organic photoreceptors include: single-layer type photoreceptors in which a charge-generating agent and a charge-transporting agent have been dispersed in a binder resin; and multilayer type photoreceptors in which functions have been allotted to a charge-generating layer and a charge-transporting layer. A feature of the latter photoreceptors, which are called the function allocation type, resides in that materials suitable for the respective functions can be selected from a wide range. Because a photoreceptor having any desired performances can be easily produced, many investigations on that type have been made.
Various improvements such as development of novel materials and combinations of these have been made in order to satisfy the performances required of photoreceptors for electrophotography, such as basic performances and high durability, as described above. However, a satisfactory photoreceptor has not been obtained so far.
Although organic materials have many merits not possessed by inorganic materials, no organic photoreceptor which satisfies all the properties required of photoreceptors for electrophotography has been obtained so far. Namely, organic photoreceptors suffer a decrease in charge potential, increase in residual potential, change in sensitivity, etc. due to repeated use and this results in deterioration in image quality. Although the causes of this deterioration have not been fully elucidated, decomposition or the like of the charge-transporting agent, etc. caused by: the active gases generating upon charge by corona discharge, such as ozone and NOX; the ultraviolet contained in the exposure light and erase light; and heat are considered to serve as some factors. Known techniques for inhibiting such deterioration include a technique in which a hydrazone compound is used in combination with an antioxidant (see, for example, patent document 1) and a technique in which a butadiene compound is used in combination with an antioxidant (see, for example, patent document 2). However, photoreceptors having satisfactory initial sensitivity are not sufficiently inhibited from deteriorating with repeated use, while ones reduced in deterioration with repeated use have problems concerning initial sensitivity and electrification characteristics. Furthermore, even the technique in which a calixarene compound is added (see, for example, patent document 3) has not produced a sufficient effect. As described above, the effect of inhibiting the deterioration has not been sufficiently obtained so far.    Patent Document 1: JP-A-1-044946    Patent Document 2: JP-A-1-118845    Patent Document 3: JP-A-5-323632